text: Let us now go back to Bernoulli:
  The velocity is the velocity of the fluid along a streamline. The streamline may not necessarily run parallel to a particular co-ordinate axis, such as the z-axis. Thus the velocity may consist of components in the axes that make up the co-ordinate system. For this analysis, we will use cylindrical polar co-ordinates . Thus .
  NOTE: We will in fact, be working in cylindrical co-ordinates for all aspects e.g. 
  Now consider the setup shown above. As before, we can break the setup into two components: upstream and downstream.